Embed Notice

@argv_minus_one @ak @shiz @erik once upon a time i was trying to play SuperTuxKart online, but for reasons at first totally mysterious to me i always timed out just when the actual race is about to begin (the lobby worked and i was able to chat). this happened on all but one server – and after lots of confusion i figured out that the reason for this is that the STK server attempts to deliver a 1442 byte packet to my computer and simply assumes that this is possible with enough retries, but in reality some router on the way always drops it, so the game gives up and dies.

in a normal network there would be no issue transferring a packet this large – not only is it smaller than the usual internet MTU, but even if it wasn’t it would simply get fragmented, or failing that the sending side would be sent an ICMP Fragmentation Needed message and would then resend the packet in sufficiently fragmented form. However my ISP fucking sucks and has put a router out somewhere with a lower MTU than everyone else and it just silently drops big packets. the thing about that one STK server that worked for me is that it has an unconventional setup – the actual STK server is somewhere behind a firewall and is port forwarded through a VPN, so its MTU is lower than usual, enough to trigger fragmentation of that evil packet at the sending side and never hit the evil router’s MTU on my side.